In the production of tufted fabric, it is well known to displace a sliding needle bar transversely of the base fabric by means of a variety of shifting apparatus. This transverse shifting may be used in order to create various pattern effects, to break up unattractive alignment of longitudinal rows of tufts, and to reduce the effects of streaking which results from variations in colorations of the yarns.
The transverse shifting of needle bars has been accomplished by the use of a variety of devices. Most of the early devices were of a cam driven type with a rotating plate cam driven directly from the tufting machine main drive shaft through a reducer, and engaging cam followers in communication with the needle bar to effect the required displacement. Because of the reliability, simplicity, and relatively low cost of cam drive systems, these systems have been in use for over fifty years and even today remain viable for use in connection with the tufting of certain carpet patterns.
Subsequently, a variety of programmable shifting mechanisms have been utilized, with the advantage that shifting patterns of these systems require only a change in programming, rather than physical replacement of a cam plate. Examples of these programmable shifting devices include pawl and ratchet devices such as are disclosed in U.S. Pat. No. 3,964,408; hydraulic shifters disclosed in U.S. Pat. Nos. 4,173,192 and 4,829,917; pneumatic shifting systems operating in substantially the same fashion as the hydraulic systems; and linear roller screw drive shifters such as are disclosed in U.S. Pat. No. 5,979,344. Each of these programmable devices suffers from some disadvantages in comparison to a cam driven system, most significantly, cost. The increased costs include not only the initial cost of purchase, but also operating costs in maintaining hydraulic or pneumatic equipment as well as the replacement of servo motors in linear drives which must absorb large forces from the needle bar.
However, the cam based systems of the prior art have numerous limitations, and thus are unsuitable for many types of patterning that might be desired. In a conventional cam driven needle bar shifter apparatus, the cam is rotatably driven through a reducing apparatus from the main shaft of the tufting machine and rotates continuously, however, since the lateral shifting of the needle bar must occur only during that portion of the machine cycle when the needles are above the base fabric and the needle plate, so as to avoid interference between the needles and the needle plate, only a portion of the cam circumference is available for controlling the needle bar movement. The remaining portion of the cam circumference is of a constant radius and non-effective for patterning, it merely idles the needle bar and is referred to as the dwell phase. For example, normally the needle bar is shifted or jogged laterally during approximately 90 degrees to 120 degrees of the needle bar reciprocation cycle, this period corresponding to the period the needles are safely free of the needle plate without imposing excessive acceleration forces on the apparatus.
Thus, in a conventional cam driven shifter approximately one quarter to one third of the circumference of the cam provides the pattern, with the remaining three quarters to two thirds of the circumference being merely an idle surface of dwell zone.
A further limitation is that if the surface of the cam is divided into sectors equal in number to the number of stitches in the pattern, the angular distance from a point in one sector to a similarly disposed point in an adjacent sector is the angle through which the cam must rotate for each revolution of the tufting machine shaft, i.e. for each cycle of the needle bar. Because of this, and because of the small surface available for a follower to ride upon each sector of a practical sized cam, the number of sectors into which the cam may be divided, and hence the number of stitches in a pattern produced by the cam, has been limited.
A further limitation upon the number of stitches in a pattern produced by cam is caused by the preferred structure of placing the rotary cam plate adjacent a sliding carrier member in communication with the needle bar, the carrier having a pair of spaced cam followers arranged to engage diametrically opposed portions of the cam. While this arrangement is perfectly satisfactory for shifting, it does have the limitation that the use of two cam followers necessitates a symmetrical cam. In turn, this produces movements of the sliding needle bar which are symmetrical about its datum. Such a machine is therefore restricted to the manufacture of fabrics having patterns which are of a symmetrical or minor image shifting pattern. While this shortcoming has been addressed through the use of two identical cams acting upon a single cam follower as depicted in U.S. Pat. No. 4,201,143, the typical diameters of cam plates for broadloom tufting machines having reached about twenty-four to thirty inches causes such a shifting mechanism to consume a great deal of space adjacent to at least one end of the tufting machine.